Using your mouse, you can draw a rubberband between the light-colored
check in the middle of the shadow and any other check. This allows you
to compare the color of different checks over the image. Alternatively
(by clicking on the button at the bottom of the applet), you can move
around a check that is exactly the color of the middle light-colored
check, and also compare its color to that of other checks.

The Illusion

The
light-colored check in the middle of the shadow is the same shade of
gray as the dark checks outside the shadow. Really!!! This illusion
was recently discovered by MIT professor Edward Adelson.

How It Works

Your visual system needs to determine the color of objects in the world.
In this case, the problem is to determine the gray shade of checks on the
floor. Just measuring the light coming from a surface (the luminance) is
not enough: a cast shadow will dim a surface, so that a white surface in
shadow may be reflecting less light than a black surface in full light.

Your visual system uses several tricks to determine where the shadows are
and how to compensate for them, in order to determine the shade of gray
paint that belongs to the surface.

The first trick is based on local contrast. In shadow or not, a check that
is lighter than its neighboring checks is probably lighter than average,
and vice versa. In this figure, the light check in shadow is surrounded by
darker checks. Thus, even though the check is physically dark, it is light
when compared to its neighbors. The dark checks outside the shadow,
conversely, are surrounded by lighter checks, so they look dark by
comparison.

A second trick is based on the fact that shadows often have sharp edges,
while paint boundaries (like the checks) often have sharp edges. Your
visual system tends to ignore gradual changes in light level, so that it
can determine the color of the surfaces without being mislead by the
shadows. In this figure, the shadow looks like a shadow, both because it
is fuzzy and because the shadow casting object is visible.

The paintness of the checks is aided by the form of the X-junctions formed
by 4 abutting checks. This type of junction is usually a signal that all
the edges should be interpreted as changes in surface color rather than in
terms of shadows or lighting.

As with many so-called illusions, this effect really demonstrates the
success rather than the failure of your visual system. Your visual system
is not very good at being a physical light meter, but that is not its
purpose. The important task is to break the image information down into
meaningful components, and thereby perceive the nature of objects in view.

Thanks to professor Edward Adelson for his kind permission to reprint this
illusion and for his explanation.